On underwater structures and on ship's hulls which are exposed to sea and/or fresh water, attachment and growth of marine organisms cause severe economic losses because of the increased friction and therefore increased consumption of fuel, or increased resistance to waves or currents (for static structures such as off-shore rigs), and because of decreased possible operation time. Traditionally major users of ships have attempted to resist fouling by painting hulls with paints containing e.g. copper or tributyltin, a tin-based compound. These paints are highly toxic and the toxic substances leach into the water, killing marine life.
Over the years several types of biocide-free antifouling paints have entered the market to meet the growing needs for environmentally acceptable alternatives. Some interest is also given to fibre coatings. By planting pines or thin fibres on a surface of marine material, settling of fouling organisms is deterred. The flexible fibres naturally sway in the water, thus creating a swaying fur-like surface, while it is said that spores, zoospores or larval organisms of algae and shellfishes generally plant particularly to relatively hard and scarcely swaying matter.
According to B. T. Watermann et al. “Bioassays and selected chemical analysis of biocide free antifouling agents” Chemosphere 60 (2005) 1530-1541, a fibre coating typically consists of short fibres in a dense pattern (200-500 fibres/mm2). First, an adhesive is applied, which serves as glue for the fixation of fibres. The fibres are then electrostatically charged and sprayed into the wet adhesive layer so that they remain perpendicularly oriented in the adhesive.
EP-A312.600 relates to a fur-like fibre-coated marine material preventing attachment of algae and shellfishes, which material is characterized in that it is covered by a number of piles comprised of a fine fibre of 5 denier or less. A denier unit is sometimes used to express the fineness of a yarn, and equals the mass in g per 9 km of yarn. It is mentioned in EP-A-312.600 that a denier greater than 5 is undesirable, because the fibre would sway less in water and further produce an uneven surface, which would attract the attachment of algae and shellfishes. Instead, it teaches the use of fibres of 1.5 denier or less, because of reasons of effect, ease of production, economic reasons and ease of handling. The effect of these small fibres on adherence of algae and shellfishes is believed to relate to the swaying properties of the fibres. The ability of the piles to sway by tidal current and wave would be controlled by the length and thickness of the piles.
Further, EP-A-353.095 discloses an antifouling sheet comprising fibre elements which are about 10-300 μm in diameter and about 10-30 mm long. The fibre elements are relatively long and the ratio of thickness versus length is selected such as to obtain a flexible fur-like surface which enables free swaying movements in water.
WO-A-93/25432 teaches the use of fibres flock with a high density of thin, short fibres, for antifouling of marine constructions. It mentions a density of 50-300 fibres/mm2, a fibre thickness of less than 0.1 mm, and a pile length of 0.5-5 mm, but is silent on any other physical property of the fibre flock, and on the process for obtaining such fibres. It is at least clear that with current techniques and materials it is not possible to produce fibres having combinations of densities and thicknesses in all parts of the ranges mentioned therein. The minimum density of 50 fibres/mm2, and the preferred minimum density of 150 fibres/mm2 indicates that very thin and short fibres are proposed, of the order of maximum 1 mm in length and a thickness smaller than 10 denier.
None of these existing swaying fur-like coatings fully prevent attachment and growth of barnacles, algae, seaweeds, shellfish and the like. Especially growth of algae and other soft fouling species cannot be reduced with similar efficiency as for instance achieved with biocidal paints.